1. Field of the Invention
The present invention relates to nitride semiconductor laser devices having a lasing threshold value corresponding to a low current value, and having an extended longevity, and semiconductor laser apparatus therewith.
2. Description of the Background Art
Jpn. J. Appl. Phys. Vol. 38 (1999) pp. L184-186 reports a nitride semiconductor laser having a GaN substrate with crystal growth thereon. The GaN substrate was prepared using the facet-initiated epitaxial lateral overgrowth (FIELO) technique and the nitride semiconductor laser device had a ridge-geometric stripe with a width of 3 μm. The document does not refer to the thickness of a residual film of the ridge-geometric stripe.
While the GaN substrate had a crystal defect density smaller for example than a conventional sapphire substrate, the device had a lasing threshold value corresponding to a high current value of 144 mA (with a threshold current density of 10.9 kA/cm2). The present inventors conducted an experiment, which has revealed that the high current value corresponding to the lasing threshold is attributed to the fact that the ridge-geometric stripe had an inappropriate width W and a residual film of the ridge-geometric stripe had an inappropriate thickness d. Furthermore, there is a corelation between the values of width W and thickness d and if at least one of width W and thickness d has an inappropriate value the current value corresponding to the lasing threshold value would not be reduced.
Herein, width W of the ridge-geometric stripe refers to a maximal width of the stripe as seen from a resonator's end surface of the semiconductor laser, as shown in FIG. 4. As such, if a ridge-geometric stripe 130 as seen in cross section is a trapezoid, as shown in FIG. 1, width W of the ridge-geometric stripe corresponds to the width of the bottom side of the trapezoid (see FIG. 1). Furthermore, thickness d of the residual film of the ridge-geometric stripe will refer to the thickness as measured from an interface of a light emitting layer 46 and a p-type layer 45 to the bottom of the ridge-geometric stripe, as shown in FIG. 4. In FIG. 4, light emitting layer 46 underlies p-type layer 45 and overlies an n-type layer 47.
Furthermore in the present specification a light emitting layer generally refers to a layer formed of a plurality of well layers and a plurality of barrier layers. It should not noted, however, that if a single-quantum well structure is applied a light emitting layer is formed of a single well layer.
The present invention contemplates a nitride semiconductor laser device grown on a nitride semiconductor substrate, wherein the aforementioned width W of the ridge-geometric stripe and thickness of the residual film of the ridge-geometric stripe have their respective, appropriate values to allow a lasing threshold value corresponding to a current value reduced to provide laser operation over a long period of time.